ORGANIC LETTERS
Synthesis and Crystal Structures of Isolable Terminal Aryl Hexatriyne and Octatetrayne Derivatives: Ar-(CtC)nH (n ) 3, 4)
2008 Vol. 10, No. 14 3069-3072
Changsheng Wang, Andrei S. Batsanov, Kara West, and Martin R. Bryce* Department of Chemistry, Durham UniVersity, Science Site, Durham, United Kingdom DH1 3LE
[email protected] Received May 15, 2008
ABSTRACT
Unprecedented stability has been observed in terminal aryl hexatriyne and terminal aryl octatetrayne derivatives by judicious choice of a bulky, nonplanar headgroup [viz., 4-(3,6-di-tert-butyl-N-carbazolyl)phenyl] which hinders topochemical intermolecular interactions in the crystal lattice.
It is generally accepted that alkyl and/or aryl terminal oligoynes with more than two conjugated triple bonds are unstable to routine isolation and purification, and these compounds decompose quickly under ambient conditions.1 Phenylhexatriyne was reported to explode at 0 °C in the absence of air during attempted isolation,2 while the unsubstituted hexatriyne exploded violently upon exposure to air.3 Kloster-Jensen et al. reported the isolation of the parent butadiyne, hexatriyne, octatetrayne, and even decapentayne [H-(CtC)nH] (n ) 2-5). However, the only data to support the structures were UV-vis spectra and 1H NMR spectra of mixtures.4 Since the instability of oligoynes increases (1) Morisaki, Y.; Luu, T.; Tykwinski, R. R. Org. Lett. 2006, 8, 689. (2) Armitage, J. B.; Entwistle, N.; Jones, E. R. H.; Whiting, M. C. J. Chem. Soc. 1954, 147. (3) Armitage, J. B.; Cook, C. L.; Jones, E. R. H.; Whiting, M. C. J. Chem. Soc. 1952, 2010. 10.1021/ol801121p CCC: $40.75 Published on Web 06/13/2008
2008 American Chemical Society
dramatically as the sp-carbon chain elongates, aryl terminal hexatriynes and higher homologues have normally been obtained as solutions rather than isolated as their pure forms.5 Organometallic analogues are generally considered to be significantly more stable than their organic counterparts,6 thus terminal organometallo-hexatriynes have been isolated and (4) (a) Kloster-Jensen, E. Angew. Chem., Int. Ed. 1972, 11, 438. (b) Kloster-Jensen, E.; Haink, H.-J.; Christen, H. HelV. Chim. Acta 1974, 57, 1731. (5) (a) Eastmond, R.; Walton, D. R. M. J. Chem. Soc., Chem. Commun. 1968, 204. (b) Eastmond, R.; Walton, D. R. M. Tetrahedron 1972, 28, 4591. (c) Johnson, T. R.; Walton, D. R. M. Tetrahedron 1972, 28, 5221. (d) Luu, T.; Morisaki, Y.; Cunningham, N.; Tykwinski, R. R. J. Org. Chem. 2007, 72, 9622. (6) (a) Dembinski, R.; Lis, T.; Szafert, S.; Mayne, C. L.; Bartik, T.; Gladysz, J. A. J. Organomet. Chem. 1999, 578, 229. (b) Dembinski, R.; Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A. J. Am. Chem. Soc. 2000, 122, 810. (c) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz, J. A. Chem.-Eur. J. 2003, 9, 3324.
well characterized.6,7 A Zn porphyrinylphenylhexatriyne has also been isolated.8 Recently, we reported the synthesis and X-ray crystal structures of a series of terminal aryl/heteroaryl butadiynes which were stable to ambient isolation and characterization.9 The crystal packing motifs of these molecules played important roles in their solid-state stability. Intermolecular cross-linking of triple bonds is a major pathway for the decomposition of oligoynes.10 Bulky aryl head groups should stabilize longer sp homologues by hindering the topochemical intermolecular interactions in the crystal lattice, which are required for the head-tail π-π cross-coupling.11 With the aim of synthesizing longer all-organic terminal oligoynes, we chose N-phenylcarbazole as a headgroup because the N-phenyl substituent cannot be coplanar with the carbazole moiety due to steric repulsion between peri-H atoms, as proven by AM1 calculations (showing potential minima at twist angles θ ) 45° and 135° with a torsion barrier of 11 kcal/mol at θ ) 0°),12 NMR data, and crystal structures of N-phenylcarbazole (θ ) 78° and 55°),12 N-(4cyanophenyl)carbazole (θ ) 48°),13 and 8 (θ ) 47°).14 We now report the successful synthesis, isolation, and X-ray structural analyses of the terminal aryl hexatriyne 10 and the terminal aryl octatetrayne 7 both of which are monocapped with an all-organic group, namely, 4-(3,6-ditert-butyl-N-carbazolyl)phenyl. To the best of our knowledge, a pure terminal octatetrayne has not been reported previously. The trimethylsilyl (TMS) protected decapentayne homologue 12 has also been isolated, although the corresponding terminal pentayne 13 was unstable to isolation. Caution: The stability of terminal oligoynes is dependent on the nature of the end groups. Although no explosions were encountered during this work, great care should be taken when carrying out similar reactions. Our synthetic strategy toward the terminal oligoynes (Scheme 1) was based on iterative cross-coupling reactions (7) (a) Hartbaum, C.; Fischer, H. J. Organomet. Chem. 1999, 578, 186. (b) Xu, G.-L.; Wang, C.-Y.; Ni, Y.-H.; Goodson, T. G., III; Ren, T. Organometallics 2005, 24, 3247. (8) Vail, S. A.; Krawczuk, P. J.; Guldi, D. M.; Palkar, A.; Echegoyen, L.; Tom, J. P. C.; Fazio, M. A.; Schuster, D. I. Chem.-Eur. J. 2005, 11, 3375. (9) (a) Wang, C.; Pa˚lsson, L.-O.; Batsanov, A. S.; Bryce, M. R. J. Am. Chem. Soc. 2006, 128, 3789. (b) West, K.; Wang, C.; Batsanov, A. S.; Bryce, M. R. J. Org. Chem. 2006, 71, 8541. (10) Klinger, C.; Vostrowsky, O.; Hirsch, A. Eur. J. Org. Chem. 2006, 1508. (11) (a) Baughman, R. H. J. Appl. Phys. 1972, 43, 4362. (b) Enkelmann, V. Chem. Mater. 1994, 6, 1337. (c) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Dougherty, D. A.; Grubbs, R. H. Angew. Chem., Int. Ed. 1997, 36, 248. (d) Xiao, J.; Yang, M.; Lauher, J. W.; Fowler, F. W. Angew. Chem., Int. Ed. 2000, 39, 2132. (12) Avendano, C.; Espada, M.; Ocana, B.; Garcia-Granda, S.; del Rosario Diaz, M.; Tejirina, B.; Gomez-Beltran, F.; Martinez, A.; Elguero, J. J. Chem. Soc., Perkin Trans. 2 1993, 1547. (13) Saha, S.; Samanta, A. Acta Crystallogr. 1999, C55, 1299. (14) Lin, H.-Y.; Zhang, X.-B.; Li, P. Acta Crystallogr. 2005, E61, o3597. (15) (a) Graaf, W. D.; Smits, A.; Bowsma, J.; Koten, G. v. Tetrahedron 1988, 44, 6699. (b) Holmes, A. B.; Jones, G. E. Tetrahedron Lett. 1980, 21, 3111. (16) Dabdoub, M. J.; Dabdoub, V. B.; Lenarda˜o, E. J. Tetrahedron Lett. 2001, 42, 1807. (17) Liu, Y.; Nishiura, M.; Hou, Z. J. Am. Chem. Soc. 2006, 128, 5592. (18) Alternative syntheses of diarylpolyynes. (a) Luu, T.; Elliott, E.; Slepkov, A. D.; Eisler, S.; McDonald, R.; Hegmann, F. A.; Tykwinski, R. R. Org. Lett. 2005, 7, 51. (b) Simpkins, S. M. E.; Weller, M. D.; Cox, L. R. Chem. Commun. 2007, 4035. 3070
Scheme 1. Synthesis of the Terminal Aryl Oligoynes
of terminal 4-(3,6-di-tert-butyl-N-carbazolyl)phenyl alkynes with 1-iodo-4-trimethylsilyl butadiyne (TMSBI)15 under Sonogashira conditions, apart from the butadiyne derivative 4 which was prepared via precursor 3 using a 2-methyl-3,5hexadiyn-2-ol protocol.9,16 For the synthesis of the TMSprotected hexatriyne 9, both methods, namely, the crosscoupling of the monoalkyne 817 with TMSBI and the cross-coupling of the iodo-butadiyne 5 with trimethylsilylacetylene (TMSA), gave 9 in comparable yields. However, the former route was synthetically preferable as the cleaner reaction mixture facilitated purification. Furthermore, the monoalkyne 8 and its counterpart TMSBI are more readily available than the diyne homologue 5. The terminal hexatriyne 10 was obtained by deprotecting 9 with potassium carbonate in MeOH-CH2Cl2 at room temperature. Similarly, the tetrayne 7 was obtained by cross-coupling of diyne 4 with TMSBI, followed by deprotection of 6. However, the attempted cross-coupling of iodo-hexatriyne 11 with TMSA under Sonogashira conditions yielded exclusively the oxidatively self-coupled diaryldodecahexayne derivative 14, as dark yellow crystals which could be stored at room temperature in the dark for 6 months without any change.18 The same reaction under Hay conditions7 resulted in decomposition of 11 and self-coupling of TMSA. We, therefore, abandoned this route. Following the successful isolation of the terminal tetrayne 7, the synthesis of pentayne 13 was attempted. Terminal hexatriyne 10 was cross-coupled with TMSBI under Sonogashira conditions to afford TMS-capped pentayne 12 in 10% yield. The orange crystals of 12 were remarkably stable: a sample was stored at rt in a flask wrapped with Al foil for a week, then in a freezer (at ca. -20 °C) for several months without any observable color change. It is likely that the low yield of 12 was due to decomposition of 10 in the reaction mixture. A large amount of insoluble dark-brown solid was observed: this is characteristic of decomposed oligoynes. The Org. Lett., Vol. 10, No. 14, 2008
attempted cross-coupling of tetrayne 7 with iodo-TMSA yielded a dark brown solid from which no product could be isolated. The attempted isolation of the terminal pentayne 13 from the deprotection of 12 was unsuccessful, although the UV-vis absorption spectrum (Figure 1b) obtained from
Figure 1. UV-vis absorption spectra: (a) 1, terminal mono- (8), di- (4), tri- (10), tetra- (7), and penta- (13) ynes (1 × 10-5 M in dichloromethane); (b) comparison of the pentaynes 12 and 13. The concentration of pentayne 13 is arbitrary.
a dilute solution of the reaction mixture was consistent with the formation of 13. The deprotection resulted in an 18 nm blue shift in the oligoynic λmax which is larger than that reported for a mesityl analogue (∆λmax 8 nm in MeOH).5c However, Maldi-Tof mass spectrometry could not detect any expected signal for 13 from the solution of deprotected 12. Presumably, 13 decomposed too rapidly as the solvents were evaporated during the matrix preparation. Therefore, the synthesis of higher homologues was not attempted. The successful isolation of the series of oligoynes 4, 10, and 7 has enabled for the first time an analysis of how the acetylenic 1H NMR chemical shift changes with extending the sp-carbon chain length up to a C8H system. In marked contrast to the red shifts of the UV-vis absorption spectra, which follow the Lewis-Calvin rule,19 the additional shielding effects caused by extending a triple bond decreased dramatically as the chain length increased. The elongation from monoyne 8 (δ ) 3.20 ppm) to diyne 4 (δ ) 2.54 ppm) to triyne 10 (δ ) 2.31 ppm) to tetrayne (δ ) 2.23 ppm) resulted in 0.66, 0.23, and 0.08 ppm upfield shifts, respectively. This progressively reduced shielding effect of the additional triple bonds can also be seen from the 13C chemical shifts of the TMS-protected oligoynes [di(t-butyl)carbazolylphenyl-(CtC)n-TMS], where the δ values of the methyl-C of the TMS group are 0.02 (n ) 1),20 -0.39 (n ) 2, Figure S35), -0.51 (n ) 3, Figure S20), -0.59 (n ) 4, Figure S11), and -0.64 (n ) 5, Figure S29), and thus the upfield shifts are 0.41, 0.12, 0.08, and 0.05, respectively. (19) Lewis, G. N.; Calvin, M. Chem. ReV. 1939, 25, 273. (20) Zhu, Z.; Moore, J. S. J. Org. Chem. 2000, 65, 116. Org. Lett., Vol. 10, No. 14, 2008
When the TMS protecting groups are removed, the 13C chemical shifts of the terminal acetylenic carbons show the same trend, i.e., 77.9 (8, n ) 1), 71.7 (4, n ) 2), 69.2 (10, n ) 3), and 68.2 (7, n ) 4) ppm. This shielding effect is in agreement with the report of Ziessel et al. for pyridyl oligoynes21 but is opposite to that reported for [H-(CtC)nH] (n ) 1-4) where the acetylenic protons were deshielded by additional triple bonds.4 The solution UV-vis absorption spectra of the terminal oligoynes are shown in Figure 1. Unlike their mesityl analogues,4 the vibronic features of the low energy oligoynic bands for 4 and 10 were not clearly visible. In particular, butadiyne 4 exhibited a very intense single peak at λmax ) 349 nm with a small shoulder on the high energy side. The loss of the fine structure could be due to overlap of the n-π* transition of the N-phenylcarbazole unit (λmax ) 343 nm for 1, Figure 1a; and λmax ) 343 nm for N-phenylcarbazole in cyclohexane22) with the π-π* transitions of the oligoynic chromophores. For the tetrayne 7 and pentayne 13, the vibronic fine structures with λmax ) 348, 371, and 395 nm (for 7) and 362, 386, and 416 nm (for 13), respectively, were well-defined. In addition, the carbazole-derived high intensity π-π* transitions below 300 nm for these two compounds also became vibronic (Figure 1b). The stability of the crystalline butadiyne 4 is remarkable. A gram-scale sample was stored for many months at -20 °C without any noticeable color change. Triyne 10 decomposed faster in chloroform solution than 4. Nonetheless, an NMR sample of 10 [ca. 5 mg in CDCl3 (0.5 mL)] survived >6 h inside the spectrometer at 20 °C. A single crystal of 10 was obtained by an accelerated evaporation process. As expected, the tetrayne 7 was less stable than the triyne 10. Crystals of 7 were stable in a freezer at -20 °C in the dark, although storage at 20 °C under standard room light resulted in darkening within ca. 10 min. Predictably, the pentayne homologue 13 was the least stable in the series. The November 2007 release of the CSD23 contains no structure with a (CtC)4H moiety and only two with a (CtC)3H, viz. (tBuNO)ClC6H3(CtC)3H (which polymerized under X-rays even at 220 K, hence only the lattice parameters were obtained)24 and (PhN-C5H4N)4Ru2(CtC)3H, where the oligoyne chain may be stabilized by pπ-dπ interaction with (21) Ziessel, R.; Suffert, J.; Youinou, M.-T. J. Org. Chem. 1996, 61, 6535. (22) Piet, J. J.; Biemans, H. A. M.; Warman, J. M.; Meijer, E. W. Chem. Phys. Lett. 1998, 289, 13. (23) Allen, F. H.; Taylor, R. Chem. Soc. ReV. 2004, 33, 463. (24) Inoue, K.; Iwamura, H. AdV. Mater. 1992, 4, 801. (25) X-ray experiments: SMART 6K CCD area detector diffractometer, graphite-monochromated Mo KR radiation (λ ) 0.71073 Å), SHELXTL 6.14 software (Bruker AXS, Madison WI, USA, 2003). At room temperature, single crystals deteriorated rapidly, both in and out of mother liquor. The diffraction of 4 and 7 was weak, mean I/σ(I) ) 5.0 and 4.3, respectively. Crystal data for C30H29N (4), C34H29N (7), and C32H29N (10): M ) 403.54/ 451.58/427.56, T ) 120 K, monoclinic, space group P21/c (no. 14), a ) 19.295(2)/21.065(3)/5.824(1), b ) 5.7668(9)/5.6665(8)/19.988(2), c ) 22.291(3)/22.090(4)/21.420(2) Å, β ) 107.45(1)/98.48(5)/96.61(1)°, V ) 2366.2(5)/2607.9(7)/2476.9(5) Å3, Z ) 4, Dc ) 1.133/1.150/1.147 g/cm, 13441/13239/24480 reflections measured, 5416/4588/6544 unique, of which 2698/2068/4950 with I > 2σ(I), Rint ) 10.0/12.7/4.2%, R1[I > 2σ(I)] ) 5.3/6.0/4.7%, wR(F2) ) 11.8/10.5/12.8% (all data). CCDC 679671 (4), 679672 (7), 679673 (10) contain the supplementary crystallographic data for this paper, which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 3071
the σ-bonded Ru atom.7b Indeed, two-thirds (28 out of 41) of the R(CtC)2H derivatives also have the R unit linked to the butadiyne unit through a (transition) metal atom. X-ray structural studies of 4, 7, and 10 (Figure 2) showed the expected twisted conformation (θ ) 58, 50, and 51°,
Figure 2. X-ray molecular structures of 4, 7, and 10.
respectively).25 The oligoyne rods are slightly bent, the terminal bonds forming an angle of 172, 174, and 164°, respectively. In each structure, it is possible to discern arrays of parallel oligoyne rods, related by the b translations in 4 and 7 and by the a translation in 10. The translation (“stacking”) vectors of 5.77, 5.67, and 5.82 Å are inclined to the rod directions by 48, 42, and 45°, respectively. The packing does not favor polymerization. In 4 the, shortest inter-rod distances (C1···C4) of 4.30 Å exceed the threshold
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of 4 Å required for solid-state polymerization.26 The shortest distances in 7, viz., C1···C4, C2···C5, C3···C6, C4···C7, and C5···C8, of 3.73-3.88 Å, are within this limit but not topologically suitable for a regular polymerization without large displacements of entire molecules.10 Such displacements, or even smaller (but still substantial) atomic displacement occurring during a “good” topotactic polymerization of butadiynes, would be seriously hindered by the bulky and nonplanar di(t-butyl)carbazolylphenyl substituent.11a Structure 10 has both problems simultaneously, the shortest contacts C1···C4, C2···C5, and C3···C6 being 4.06-4.17 Å. In summary, unprecedented stability has been observed in all-organic terminal aryl hexatriyne and octatetrayne derivatives by judicious choice of a bulky, nonplanar headgroup [viz., 4-(3,6-di-tert-butyl-N-carbazolyl)phenyl] which serves to hinder the topochemical intermolecular interactions in the crystal lattice which are required for the head-tail cross-linking of the oligoyne units. New acetylenic building blocks which are suitable for the assembly of polyyne chains and other carbon-rich scaffolds27 have been synthesized. Acknowledgment. We thank EPSRC for funding this work. Supporting Information Available: Supporting Information for this article including synthetic details, NMR, and MS spectra. This material is available free of charge via the Internet at http://pubs.acs.org. OL801121P (26) Enkelmann, V. AdV. Polym. Sci. 1984, 63, 91. (27) (a) Carbon-Rich Compounds: From Molecules to Materials; Haley, M. M., Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, Germany, 2006. (b) Acetylene Chemistry: Chemistry, Biology and Material Science; Diederich, F., Stang, P. J., Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, Germany, 2005. (c) Nielsen, M. B.; Diederich, F Chem. ReV. 2005, 105, 1837. (d) Bunz, U. H. F.; Rubin, Y.; Tobe, Y. Chem. Soc. ReV. 1999, 28, 107.
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